Learning about words is central to learning about language. To learn about the meanings and properties of words, infants must be able to recover recognizable word shapes from the continuous input speech that they hear. The difficulty of this task is readily apparent to anyone listening to speakers conversing in an unfamiliar language: what one hears is a babble lacking readily identifiable words. The development of word-shape recognition skills is prerequisite to further acquisition of language, for if a language learner cannot break input utterances into their constituent parts, it will be impossible to learn how these parts fit together or what individual parts mean. Despite the fundamental importance of word segmentation and word-shape identification to communicative development, research has only recently begun to explicate the nature and development of requisite cognitive and perceptual capacities. The studies proposed in this application seek to address questions including: How do the perceptual abilities recruited for word segmentation across the second half of the first year interact with developing lexical representations? How and when do infants succeed in disentangling multiple influences- arising from lexical properties, discourse factors, speaker affect, talker variability, and so forth- on prosodic characteristics of word shapes? To what degree does developing knowledge of language-particular patterns of word structure modulate early word-shape identification? Are individual differences in early word-shape recognition abilities predictive of aspects of later language development? What quantities and qualities of language experience are required for development of word segmentation and word-shape identification skills? How do these skills develop in infants with congenital sensorineural mild-to-severe hearing impairment? To these ends, the research proposed here will primarily use a word recognition extension of the conditioned head turning method to investigate 6- to 14-month-old infants' segmentation of fluent natural speech.